The present invention relates to an air-fuel ratio control system for automotive vehicles, or more in particular to an air-fuel ratio control system intended to improve the running performance of the automotive engine under heavy load.
Conventionally, under the heavy load conditions with the throttle fully open, the air-fuel ratio is controlled by feedback for a predetermined length of time (say, four seconds) following the full opening of the throttle and then the feedback loop is opened thereby to stop the air-fuel ratio control. In this conventional air-fuel ratio control, even though the throttle is opened full for acceleration, a sufficiently rich mixture gas is not obtained since the air-fuel ratio is being controlled, thus posing the problem of acceleration.